Move drush from main homebrew to homebrew-php

 * Moved drush from main homebrew to homebrew-php.

   The drush formula now requires a build dependency on composer which is
   located in homebrew-php.

 * Drush HEAD build depends on composer.
    
   The HEAD version of drush has a build dependency on composer.

See Homebrew/homebrew#31696 and Homebrew/homebrew#30596 for issues regarding not having the formula in the main Homebrew repository anymore.

Ping; @mikemcquaid.
Probably want these to point to `etc` and `share`, no? At least `prefix` rather than `HOMEBREW_PREFIX`.